The purpose of this project is to establish the system of assay in vitro malignant transformation of human and mouse cells by chemical carcinogens. As a continuation of our previous success in the transformation of human cells, quantitation and earlier detection of the transformation of human cells after chemical treatment have been attempted. Abortive or transient transformation was observed for one to three months after treatment with 4-nitroquinoline-1-oxide. Treatment with a promoter during one month after 4NQO-treatment enhanced the frequency and shortened the time of the development of abortive transformation. However, treatment with the promoter did not result in the enhancement of stable and malignant transformation. A quantitative system for assay of malignant transformation using a subclone of BALB/3T3-A31 clone was improved and was given to many outside laboratories.